Multiple blade mowing decks are of either the side discharge type or a rear discharge configuration, such as taught in applicant's U.S. Pat. No. 4,916,887. There are certain advantages to a rear discharged deck such as the safety factor of better shielding from objects thrown from the blades. A rear discharge deck also permits the mower to trim close to an obstruction on either side, rather than a single side as is the case with the side discharge mower, which has an extending discharge chute on one side. Also the clippings are deposited on the cut swath and not onto undesirable areas such as a sidewalk or drive.
The advantage of a side discharge deck is that it provides a much better quality of cut by reason of the rear skirt which wraps around each of the cutting blades and provides a maximum suction or lifting of the grass along the back half of each blade chamber.
As a rotary blade and deck move over uncut grass, there is an initial cutting as the blade sweeps across the front half of the deck. The second cutting of that same area takes place as the blade swings across the rear half of the deck. It is important to have good suction or lift across the rear half of the deck to avoid leaving uncut grass. With a conventional rear discharge deck, the air and grass clippings flowing out the rear portion of the skirt diminish the quality of the cut achieved by the rear half of the deck.
Most rotary cutting blades not only provide a cutting edge for the grass, but they also provide an upturned trailing edge of the blade acting as an axial flow fan to lift the air and clippings from the plane of rotation from the blade and directing the clippings into a discharge chute for collection. The turned up rear edges of the cutting blades, referred to as sails, provide the basic suction of air and clippings through the bottom of the deck in conventional decks. In cutting decks with multiple blades, the pattern of air flow and clippings will vary with designs, as illustrated in U.S. Pat. No. 4,114,353 and in the Applicant's above mentioned '877 patent.
In many multiple blade decks, the combined lifting force of the sails on the multiple blades does not produce sufficient suction to lift the clippings into a hopper, and a secondary fan device is required as typically shown in the U.S. Pat. No. 3,657,865 along with the above mentioned patents.
In single blade rotary mowers, the concept of using a secondary fan over the cutting blade is taught in numerous forms through the use of a concentrically mounted fans in U.S. Pat. Nos. 4,263,771; 4,407,112; 5,012,633; 4,121,405; 3,696,595 and Australian Patent No. 264,584.
The rear discharge multiple blade decks of the prior art take the clippings discharge through the rear skirt of the deck, as taught in applicant's above mentioned '887 Patent, which diminishes the quality of the cut since there is no longer strong suction along the rear skirt.